bpo-38041: Refine IDLE Shell restart lines.

Lib/idlelib/NEWS.txt

@@ -3,6 +3,10 @@ Released on 2019-10-20?
 ======================================
 
 
+bpo-38401: Shell restart lines now fill the window width and always
+start with '='.  Filenames still wrap if long relative to the width,
+but there is no longer a trailing space that can wrap by itself.
+
 bpo-37092: Add mousewheel scrolling for IDLE module, path, and stack
 browsers.  Patch by George Zhang.
 

Lib/idlelib/idle_test/test_pyshell.py

@@ -7,6 +7,28 @@
 from tkinter import Tk
 
 
+class FunctionTest(unittest.TestCase):
+    # Test stand-alone module level non-gui functions.
+
+    def test_restart_line_wide(self):
+        eq = self.assertEqual
+        for file, mul, extra in (('', 22, ''), ('finame', 21, '=')):
+            width = 60
+            bar = mul * '='
+            with self.subTest(file=file, bar=bar):
+                file = file if file else 'Shell'
+                line = pyshell.restart_line(width, file)
+                eq(len(line), width + 1)  # +1 for '\n'
+                eq(line, f"\n{bar+extra} RESTART: {file} {bar}")
+
+    def test_restart_line_narrow(self):
+        expect, taglen = "\n= RESTART: Shell", 16  # Don't count '\n'.
+        for width in (taglen-1, taglen, taglen+1):
+            with self.subTest(width=width):
+                self.assertEqual(pyshell.restart_line(width, ''), expect)
+        self.assertEqual(pyshell.restart_line(taglen+2, ''), expect+' =')
+
+
 class PyShellFileListTest(unittest.TestCase):
 
     @classmethod

Lib/idlelib/pyshell.py

@@ -387,6 +387,16 @@ def handle_EOF(self):
         "Override the base class - just re-raise EOFError"
         raise EOFError
 
+def restart_line(width, filename):  # See bpo-38141.
+    if filename == '':
+        filename = 'Shell'
+    tag = f"= RESTART: {filename} ="
+    if width >= 13 + len(filename):  # The length of tag.
+        div, mod = divmod((width -len(tag)), 2)
+        return f"\n{(div+mod)*'='}{tag}{div*'='}"
+    else:
+        return '\n' + tag[:-2]  # Remove ' ='.
+
 
 class ModifiedInterpreter(InteractiveInterpreter):
 
@@ -491,9 +501,7 @@ def restart_subprocess(self, with_cwd=False, filename=''):
         console.stop_readline()
         # annotate restart in shell window and mark it
         console.text.delete("iomark", "end-1c")
-        tag = 'RESTART: ' + (filename if filename else 'Shell')
-        halfbar = ((int(console.width) -len(tag) - 4) // 2) * '='
-        console.write("\n{0} {1} {0}".format(halfbar, tag))
+        console.write(restart_line(console.width, filename))
         console.text.mark_set("restart", "end-1c")
         console.text.mark_gravity("restart", "left")
         if not filename:

Misc/NEWS.d/next/IDLE/2019-09-05-23-12-13.bpo-38041.nxmGGK.rst

@@ -0,0 +1,3 @@
+IDLE Shell restart lines now fill the window width and always start with
+'='.  Filenames still wrap if long relative to the width, but there is
+no longer a trailing space that can wrap by itself.